VOLUMINOUS MID-MIOCENE SILICIC VOLCANISM AND RAPID EXTENSION IN THE SIERRA LIBRE, SONORA, MEXICO

Preliminary field and ⁴⁰Ar/³⁹Ar data from Sierra Libre (SL) and surrounding areas reveal a period of intense volcanic activity directly followed by a short period of rapid extension.  Rocks of the SL include scattered 20 to 13 Ma andesites and basalts, overlain by a >1 km thick section of 13 to 12 Ma rhyolite and dacite lavas and minor tuffs, locally overlain by flat-lying post 11 Ma basalts.  The total volume of silicic volcanic rocks is >5000 km³, yet no major ignimbrites have been identified.

            Imbricate steep normal faults cut and tilt the section 30° to 40°.  Preliminary cross sections and structural relations in the SL suggest 20% to 50% extension occurred between 12.06 ± 0.02 Ma (youngest tilted unit) and 10.75 ± 0.2 Ma (oldest flat-lying basalt).  This angular unconformity likely correlates with the ~11 Ma unconformity identified by Mora-Alvarez and McDowell (GSA Special Paper 334, p.123, 2000) in the Sierra Santa Ursula.  Younger sub-vertical, NNW-striking strike slip faults have been identified in the Sierra Libre, but their amount, timing, and sense of offset is not yet clear.

            The inception of extension (11 to 12 Ma) in the coastal part of the Sonoran rifted margin correlates well with the plate tectonic change from subduction to transform motion at this latitude and may have accommodated part of North American-Pacific plate motion.  However extension is largely restricted to the coastal belt and was active only during the earliest stages (9 to 12 Ma) of the proposed "proto-gulf" deformation.  Preliminary investigations in interior Sonora indicate that significant extension occurred prior to 12 Ma, and that this extension gets older farther east (Blair et al., 2003, Gans et al., 2003, Wong et al., 2003, all this volume).  Farther west, in Sierra El Aguaje, major extension appears to be slightly younger (~9 to 10 Ma).  Ongoing work in the Sierra El Bacatete (the next range east) may reveal whether extension in the Sierra Libre is separate in space and time from extension farther east, or whether it is only the youngest manifestation of a westward migration of extension since ~25 Ma.